This section was edited by Associate Editor Alan S. Brown.

POWER TRANSMISSION & MOTION CONTROL

DRIVER IN A BOTTLE CAP

THE PUCK IS A BOTTLE CAP-SIZE DRIVER THAT WEIGHS LESS THAN 50 GRAMS, YET IT CAN CONTROL FRACTIONAL HORSEPOWER SERVOMOTORS UP TO 300 WATTS. Originally designed for robotic arms, it plugs directly into the robot’s brushless servomotors and includes a magnetic array encoder for precise positioning.

The technology is intended to eliminate the large drive cabinets and scores of wires used to control most robots. The Puck comes with its own encoder, plugs directly into the motor, and uses only two power and two control wires, said developer Bill Townsend, CEO of Barrett Technology Inc. in Cambridge, Mass.

Townsend developed the Puck for his WAM robot arm, which has seven degrees of freedom plus four motors in its hand. It originally took a 230-volt, 3-phase, 20-amp/phase power supply to drive the arm. Using Pucks, it now runs on a notebook computer power brick.


Barrett Technology’s tiny Puck driver plugs into a servo and replaces controllers ordinarily housed in a cabinet.

The Puck eliminates the 100 or more wires required to drive today’s sophisticated robot arms. “I’ve been building robots for 24 years, and the number-one hardest part about designing the physical robot is the wiring. Anyone in the industry will tell you the same,” Townsend said.

Robots have so many cables that they need external guides to route them around the arm. This limits joint range, especially in human-scale robots, and both guides and cables are vulnerable to snagging and working loose. According to Townsend, the Puck’s simplified wiring can double a small robot’s effective range of motion.

Townsend’s Puck is a major break with existing technology. Today’s robot motors use encoders to determine the position of the rotor. Drivers use that information to determine the amount and phase of power they should send to each motor electromagnet to reach the desired position and torque. All measurements and calculations take place at blindingly fast speeds while the motor spins at 5,000 revolutions per minute.

Conventional controllers use a power supply that switches power transistors (typically two for each phase) on and off roughly 25,000 times per second. This generates lots of electrical noise as currents jump between components and wires. Engineers traditionally solved this problem by putting the drivers in a big box, far away from the robot, and adding heavy-duty circuitry (with its own power supplies and cooling fans) to isolate the sensitive circuits from the noisy ones.

According to Townsend, he was at MIT when the first power transistors were developed, and he often wondered if there was a simpler way to incorporate them into drives. He said he found that shrinking component size, using a copper foil to separate noisy from sensitive components, and minimizing wire lengths eliminated most electromagnetic interference.

No noise meant no need for noise-reduction circuitry or long cables from the encoder to isolated drive cabinets. Townsend could combine the driver and encoder into a package small enough to place on a servomotor. Townsend said the package is so efficient the he can encase all the electronics in epoxy and draw off heat using conduction cooling.

Townsend is already using the Puck to operate his WAM robot arm, which is currently undergoing testing by General Motors and several other organizations. Meanwhile, he has received several National Science Foundation grants to find more cost-effective ways to manufacture the Puck. He received his first U.S. patent (No. 7,511,443) in March 2009 and has two more pending. His goal is to slash production costs so that the Puck can be used to drive servomotors in a broader range of robots and industrial systems.

MICROSTEP EFFICIENCY

Recessions drive engineers to create products that are cheaper and more efficient. QuickSilver Control Inc., a small Covina, Calif., motor and driver manufacturer, says it has hit both marks with its new NEMA 23 servomotor.

QuickSilver occupies a special niche in servomotors. Its drives convert microstep motors to servomotors (which are actually closed loop stepper motors). They do this by using a four-quadrant vector controller (similar to a variable-frequency drive) that drives their 2-phase ac motors like servomotors.


QuickSilver controllers let 2-phase NEMA motors act like servomotors.

According to QuickSilver, its controller gives the microstep motors the power and precision of servos plus high torque while they retain the low cost of steppers. Servo control also keeps the stepper motors running cooler and prevents stalling.

According to the company, the QIL-A23L-3 NEMA 23 achieves nearly one-third more power output and 15 percent more efficiency over the speed range of the motor it replaces. The key improvement involves a new motor winding that reduces heat loss. The motor also has a new encoder that doubles the counts per revolution to 8,000.

The motor itself runs at 4,000 revolutions per minute. It produces 84 ounce-inches (or 0.59 newton-meters) of continuous torque, 124 ounce-inches (0.86 newton-meters) of peak torque, and 80 watts of continuous mechanical power.

Even more impressive, it handles up to 100:1 inertial mismatches, so it can drive large flywheel loads without additional gearing. QuickSilver sees applications in semiconductor manufacture, packaging, medical sample handling, and food processing.

QuickSilver charges $295 for individual motors (and $495 for the recommended SilverDust driver). Prices drop significantly with volume.

ACTUATORS FROM PAPER

Imagine making a compass from a piece of newspaper, then using the same material for actuators soft enough to manipulate cells without breaking their delicate membranes. Those are just some of the possibilities of ferropaper, ordinary paper infiltrated with inexpensive ferrofluids, developed by Babak Ziaie, a professor of electrical engineering at Purdue University’s Birck Nanotechnology Center.

Ferrofluids are commercially available, low-cost oil or water suspensions of 10 to 20 nanometer diameter iron particles. When Ziaie pours the suspensions over paper, the iron particles soak into the paper’s pores. This turns the paper into a ferromagnetic iron composite.

Ziaie then has the option of coating it with parylene, a vapor-deposited polymer with excellent moisture barrier properties. Add one or two layers of parylene and ferropaper is soft and pliant enough to move fragile objects. Add more coating layers and it becomes harder and stiffer.

The higher the suspension’s iron loading, the more ferromagnetic the paper. Like iron, it can be manipulated or magnetized by magnetic fields.


Impregnating paper with iron particles enables these magnetically controlled birds to flap their wings.

Ziaie has used this property to create ferropaper cantilevers and linear actuators. The cantilevers are simple devices that move up and down. The linear actuators are accordion-like structures roughly 3 millimeters wide that he can pull with an electromagnet. Turn off the magnet and the actuator snaps back. The more parylene coatings on the actuator, the faster it returns to its original position.

Ziaie makes the accordion structures by cutting the paper with lasers. “We can make more complicated shapes as well, likes springs, zigzags, and tubes,” he explained. “They are all lightweight, and we can control their responsiveness and stiffness.”

The new material has Ziaie thinking about biomedical applications, such as small surgical tools. “There are applications, like eye surgery, where you don’t want to exert too much force. Perhaps we could design a coil to go around the ferropaper so it acts like a plunger but only generates a limited amount of force,” he said.

Ferropaper could also find its way into tools to measure cell properties. “There’s a lot of interest in in-vitro manipulation of cells because of recent research that finds that mechanical properties play a role in whether cancers metastasize or stem cells grow,” he explained. “If we apply stress to stem cells, we might be able to force them to become bone or muscle preferentially.”

KNOCKDOWNS HELP CALCULATE BEARING STRESS
By Fred Finkelstein

DESIGNING BEARINGS SEEMS A STRAIGHTFORWARD MATTER OF SIZING PINS AND SHAFTS. Yet the load placed on mating plates is also significant. Calculating only shear and breakthrough loads misses a problem common with high-cycle mating plate designs. This is the gradual elongation of a hole toward a load over time, a process called wallowing.

One way to attack wallowing (as well as pin failures) is to find the endurance limit of the bearing material. The endurance limit uses empirical evidence on wear to adjust a material’s tensile strength downwards through a series of “knockdowns” that provide a truer picture of how a material stands up to long-term fatigue.

Take, for example, a pin and matching plate made of AISI 4130 carbon steel with tensile strength of 80,000 psi and shear strength of 28,000 psi. Assuming the mating plate is 0.5 inch thick and must withstand repeating/reversing loads of 1,000 psi, how thick should we make the pin?

Using a material’s endurance limit helps engineers avoid wallowing (shown in red), the elongation of a hole towards the load on a mating plate.

We could derive the pin diameter using stress equals force/area (S=F/A). The allowable stress would be the shear strength of 28,000 psi, and the force 1,000 psi. The area would equal the thickness of the mating plate (0.5 inches) multiplied by the diameter of the pin. This would yield a pin diameter of 0.072 inch. Apply a safety factor of two and the pin diameter is still less than 0.15 inch in diameter.

The endurance limit produces very different results. The calculations apply such knockdown factors as surface finish, size effects, temperature, and stress concentrations to tensile (break) strength. In AISI 4130 steel, this produces a corrected endurance limit of 11,120 psi.

Then comes another calculation informed by empirical evidence. When calculating the area of the pin, include only the 60 percent of the pin diameter (the middle cross-section minus top and bottom) that transfers force to the mating plate. When combined with the endurance limit, this produces a pin diameter of 0.30 inch. This is four times thicker than the original calculation, enough to prevent fatigue, wallowing, and pin deformation.

Material properties may change significantly with temperature. For example, the shear strength of AISI 316 stainless steel at room temperature is 12,500 psi. At 1,000 °F, however, it falls to 12,000 psi after 10,000 hours and to 7,500 psi after 100,000 hours. Starting with temperature-corrected properties before applying knockdown factors ensures a material will maintain its original safety factors and not creep or fail over its service life.

Some designs use a bushing or flange bearing pressed into the link arm. These parts are often made of bronze or hardened brass, which have lower yield strength than the material used in the link or pin. When calculating maximum stresses for these material stacks, use the properties of the weakest material as the starting point for the endurance limit.

Finally, do not forget the pin itself. Engineers need to double-check pin material, diameter, and clamp load to validate fully the link connection.


Editor’s note: Fred Finkelstein is a mechanical engineering consultant at Iron Designer, Diamond Bar, Calif.

DRIVE SPECIALIZES IN PUMPS, FANS

As electronics have grown smarter, they have learned to excel at a broad range of tasks. Modern PLCs provide features once found only in high-end automation applications. Today’s laptop is an all-purpose measurement tool. Yet many companies are using today’s more powerful processors to master a limited number of well-defined tasks.

ABB Automation Products’ new ACS310 low voltage drive is a good example of this trend. It was designed for extremely fast installation and commissioning, especially for pumps and fans. It combines specialized pump and fan functions with energy management, small size, and a 0.5 hp to 30 hp (0.37 kW to 22 kW) power range.

The unit provides several specialized features. Its pump and fan control feature modulates main pump or fan speeds, and it can bring auxiliary units online as needed while equalizing their working hours over time.

The drive supervises inlet and outlet pressures, and detects underload and overload conditions. It provides soft starts and stops for pipe filling, gradually ramping motors up to required speed in order to reduce mechanical stress and water hammer effects. There is also a built-in pump cleaning sequence to prevent jamming.

The ACS310 also features an energy optimizer that minimizes motor losses and noise while reducing motor magnetizing current (especially on partial loads). A counter displays energy savings in kilowatt-hours or local currencies. ABB points out that it can be a useful feature for plants that audit energy savings for green certifications.

The drive’s smarts come from integrated proportional-integral-derivative (PID) controllers. They vary the drive’s output in response to changes in pressure, flow, or other external information. Not only do they eliminate the need for an external programmable controller, but they save energy (and money) by driving the fan or pump only as hard as necessary.

To keep down costs, ABB does not supply the drive with a control panel, which the company says is needed only for commissioning or troubleshooting. Since the panels are detachable, ABB suggests using a single panel to troubleshoot several units. Nor does the drive support sequence programming, vector control, and dynamic braking using brake choppers.

On the other hand, the ACS310 comes with preprogrammed application macros for common tasks, supplemented by assistant screens that guide users through startup, timed functions, preventive maintenance routines, and failure diagnosis. Rather than show all parameters, the menu defaults to the most important options to ensure rapid commissioning.

The drive comes with an embedded Modbus remote terminal unit and RS-485 and RS-232 connections as standard.